1. Field of the Invention
The present invention relates to an amplitude-detecting circuit usefully applicable in extraction of amplitude values from AC signals with jitters in the periods, such as detected signals from various instrumentation sensors, for example, a touch signal probe driven by a piezoelectric element, an electrostatic capacitance gap sensor, and the like.
2. Description of Related Art
As an instrumentation sensor for use in shape measurement of a mechanical structure, for example, a touch signal probe driven by a piezoelectric element is known as shown in FIG. 6. A stylus 71 has a spherical contact 73 attached at the tip and a balancer 74 at the rear end. A stylus holder 72 holds the stylus 71 approximately at the center of the length. A piezoelectric element 75 is attached approximately at the center of the stylus 71 to impart vibrations on the stylus 71. The piezoelectric element 75 has a vibrating electrode 75a to which a driving signal is applied from a driver 78 and a detecting electrode 75b from which a mechanical-electrical converted signal is detected by a detector 76. The detector 76 positively feeds an output signal back to the driver 78. This feedback control allows the piezoelectric element 75 to be excited in a resonance state at a certain frequency. The signal detected at the detecting electrode 75b is a sine wave alternating signal in the form of an amplitude-modulated carrier (vibrating signal), of which amplitude and frequency vary when the contact 73 touches a work to be measured. A signal processor 77 is employed to check the amplitude of the signal obtained from the detector 76 in order to detect the touch.
The detected signal from the touch signal probe contains jitters in the amplitude as well as in the period influenced from non-linearity of the piezoelectric element, interference among many vibration modes caused from a complicated structure, disturbances and so forth. The frequency of the detected signal is in the proximity of the frequency of the vibrating sinusiodal wave signal but fluctuates within a certain range below and above the vibrating frequency. This is disadvantageous when the amplitude of the detected signal must be detected fast or with no time delay, and with a high precision.
A well-known conventional sampling system with a constant sampling period can be employed to detect an amplitude peak value of the signal detected by the above touch signal probe. This sampling system is possible to perform a high accurate detection when the period of the detected signal is constant but causes errors in amplitude value detection in response to a period fluctuation of the detected signal when the period is not constant. In general, the detected value varies in response to a period of the vibrating frequency.
There is a method of rectifying full waves a detected signal then passing it through a low pass filter to remove ripples. This method is often employed to extract an amplitude value of an amplitude-modulated signal but has a large time delay on amplitude extraction due to a time constant of the low pass filter. Therefore, it can not be employed in a feedback control system for real-time constant-value controlling of an amplitude value that varies time to time.
Recently, in a remarkable digital processing system, all detected information is fast sampled, then A/D converted, and a mass digital data thus obtained is stored in a mass memory for later FFT analysis and filtering. Such the processing system can detect an amplitude value with high accuracy per period component of the detected signal while it is complicated and expensive. In addition, the FFT processing for the mass digital data requires a long time. Accordingly, the above processing system is effective only for an audio system and an instrumental system, of one-directional information transmission type, which are sufficient post-processing detected data. To the contrary, it is not applicable as such to an automatic control system that essentially requires a real time processing to feedback control the amplitude value varying in time to time as described above.